The production of clean fuel gas from coal or other carbonaceous solid feedstock promises to be an economical source of energy and/or chemical feedstock for the electric generating and chemical process industries. In a typical coal gasification system, crushed or pulverized coal is admitted into a vessel along with a metered amount of oxygen or air. Partial combustion of the coal with the oxidant results in the production of a fuel gas containing combustible constituents such as carbon monoxide, methane, and hydrogen. The fuel gas as initially produced also may contain such undesirable compounds as hydrogen sulfide, inert flyash, and unreacted carbon. It is typical in such systems to remove the sulfur and ash compounds from the fuel gas stream for eventual disposal, and for economic reasons to separate and utilize the unreacted carbon, possibly by recycle to the gasifier vessel.
Pressurized coal gasification systems operate at absolute pressures of three atmospheres or higher, typically being used to supply a fuel gas to a generating gas turbine. The gasification reaction, product gas cooling, gas cleanup, and other functions of the system must all be performed under pressure, entailing a considerable increase in cost and complexity over an atmospheric system. This additional cost and complexity is offset by the higher product gas energy content and reduced equipment size resulting from the use of a pressurized process.
Typical pressurized gasification systems use a double wall vessel for the gasification and initial heat exchange steps. In a typical double wall vessel, an inner, water-cooled chamber contains and directs the reactions and product gas flow while an outer, pressure resistant chamber provides the physical strength necessary to contain the process. In this system, the inner vessel is designed to protect the outer vessel from the heat of the gasification reaction and product gas, but is unable itself to withstand a significant pressure differential across its walls. Typical chambers of this type are constructed of a plurality of longitudinally welded water carrying tubes, such as those used in modern utility steam generators.
The outer vessel, designed to contain the pressures of the gasification process, is typically a solid vessel which, without the protection of the water-cooled inner chamber, is unable to withstand the high heat flux generated by the exothermic coal-oxidant reactions. For elongated inner and outer chambers, an annular space between the water-cooled and pressure-restraining members permits access for repair and maintenance of the respective components.
Due to thermal transients experienced during operation, a differential vertical thermal expansion of 20 centimeters (8 inches) or more may occur between the inner and outer chambers of a double wall containment vessel. For systems requiring multiple feed and extraction points for feedstock, waste materials, and product gas, it is therefore necessary to accommodate the differential vertical thermal displacement at various points along the length of the double wall vessel in order to permit entry and exit of the reactants and products of the gasification reaction. Such solutions are also known in the prior art and may include sliding seals, flexible connectors and other means.
What each of these prior art methods has in common is the lack of a totally secure sealing method around the point of penetration into the inner, water-cooled chamber. Moreover, typical construction methods for fabricating inner vessels of longitudinally welded tubing often result in an inner chamber wherein material may leak between the interior of the water-cooled chamber and the surrounding annular volume, and vice versa. Such leakage is undesirable as the untreated product gas is dirty and corrosive and may adversely affect equipment within the annular volume. Alternatively, if inert nitrogen is supplied into the annular volume to create a slight differential pressure across the water-cooled chamber wall, any leakage of this seal gas into the product gas flow stream will result in a dilution of the gas stream and a degradation of the final product gas heating value.